The present invention relates to current sensors and, more particularly, to a differential technique for overcoming offset voltages in an amplifier employed to provide feedback compensation in a transformer of a current sensor.
Many electrical and electronic devices, such as induction and electronic-type watthour meters for metering electric power and energy usage, require means for sensing line or load current components flowing in a conductor, and producing a current measurement signal which is accurately proportional over a large range of magnitudes of the load current.
The load current is typically many times the value of the current measurement signal appropriate for use in an electronic metering device. In some systems, the load current is as much as 10,000 times larger than the desired current measurement signal. It is convenient to employ a transformer, such as a current transformer, wherein a relatively small number of turns (e.g., one or two) about a toroidal core serve as a primary transformer winding carrying the load current. A secondary winding of many turns has induced therein a current proportional to the load current but reduced by the primary-to-secondary turns ratio of the transformer.
Transformers are prone to core saturation in the presence of large load currents. Core saturation is generally avoided by using large cores and making the cores of high-quality materials. Unfortunately, both large size and high-quality materials result in high cost.
Prior techniques for avoiding core saturation include providing a feedback winding about the core carrying a feedback current signal just sufficient to maintain the core flux near zero. Limiting the core flux near zero permits using smaller cores and cheaper core materials. As the load current changes, the feedback current signal also changes just enough to maintain the core flux near zero so that each different level of load current can be accommodated without inducing core saturation in the transformer.
The active feedback employed in the foregoing technique is generated by an operational amplifier receiving the output of the secondary winding of the transformer. The typical high gain of an operational amplifier allows for producing an output current readily capable of maintaining near zero flux in the core. The high gain of the operational amplifier, however, leads to a further complication. As will be appreciated by those skilled in the art, coupling between the feedback winding and the secondary winding of the transformer is only effective for alternating current (AC). Them is no direct current (DC) feedback coupling to, the input of the operational amplifier. Thus, DC offset voltages of, for example, a fraction of a millivolt, may appear or develop at the input of the operational amplifier. Typical operational amplifiers have DC gains on the order of several million. As a consequence, any offset voltage, even a fraction of a millivolt, at the input of the operational amplifier can drive the operational amplifier to saturation.
U.S. Pat. No. 4,761,605, assigned to the assignee of the present invention and herein incorporated by reference, describes a feedback circuit which employs a single-ended operational amplifier and chopping switches to convert the response to any DC offset voltage into an AC component which in turn is coupled between the feedback and secondary windings of the transformer in order to provide DC compensation. Although the foregoing U.S. Pat. No. 4,761,605 is effective in providing the desired DC compensation, the feedback circuit employed therein causes discontinuous polarity reversal in the desired measurement signal and this necessitates additional synchronization or signal polarity "bookkeeping" in order to filter out or remove such discontinuous polarity reversal from the measurement signal. Further, since the feedback circuit may comprise an integrated circuit chip and the current sensor may have to handle multiple current and/or voltage interface channels, it is desirable to reduce the number of connect pins required per signal interface channel in the current sensor.